The Archduke (Daindreth's Assassin, #0.5)

Daindreth Fanduillion has learned to live with it after inheriting the curse that drove his father mad, but a sorceress’s assassination attempt changes everything. She fails, but temporarily frees him from the demon that has plagued him for more than half his life.

To his surprise, Daindreth learns that she is Amira Brindonu, half-sister to his betrothed and daughter of a sworn enemy. Guilty of high treason, she should be sent to the gallows, but Amira is his only hope and Daindreth is willing to pay any price for the chance at a cure.

Against the advice of his friends and advisors, he becomes determined to do whatever he must to win her over into his service—if she doesn’t kill him first.

Then again, losing his life seems a small price to pay for the chance to save his soul.